One of the most important classes of intracellular activities is the post-translational modification of proteins. Post-translational modification activities modify proteins within living cells to effect changes in their biological activity and/or function. Major types of protein post-translational modification include protein phosphorylation, dephosphorylation, methylation, prenylation, glycosylation, ubiquitination, sulfation, and proteolysis.
Protein modification by kinases and phosphatases is generally recognized as an important mechanism for regulating protein function. Protein kinases modify proteins by the addition of phosphate groups (phosphorylation), primarily on the amino acids tyrosine, serine, or threonine. Protein phosphatases, in contrast, act to remove these phosphate groups. Changes in the phosphorylation state of proteins can affect enzymatic activity, protein localization, and protein-protein interactions. Such changes can subsequently modulate cellular metabolism, regulation, growth, and differentiation.
Researchers have found more than 400 human diseases and disorders arising from genetic defects in protein kinases. Thus, the over 600 kinases and phosphatases encoded by the human genome represent potentially powerful targets for drugs. Current methods of measuring protein kinase activity, however, have many disadvantages, hampering the ability to rapidly screen kinases for drugs. For example, many current methods for measuring kinase activity rely on the incorporation and measurement of 32P into the protein substrates of interest. In whole cells, this necessitates the use of high levels of radioactivity to efficiently label the cellular ATP pool and to ensure that the target protein is efficiently labeled with radioactivity. After incubation with one or more test drugs, the cells must be lysed and the protein of interest purified to determine its relative degree of phosphorylation. This method requires large numbers of cells, long preincubation times, and careful manipulation and washing steps to avoid artifactual phosphorylation or dephosphorylation. Alternative kinase assay methods, such as those based on phosphorylation-specific antibodies using ELISA-type approaches, involve the difficulty of producing antibodies that distinguish between phosphorylated and non-phosphorylated proteins.
There is thus a need for assays to monitor post-translational protein modification enzymatic activities, including kinase and phosphatase enzymatic activities, that are sensitive, simple to use, and adaptable to high-throughput screening methods.